1. Field of the Invention
This invention relates to an apparatus for controlling a line pressure used in a V-belt type continuously variable transmission for an automobile (hereinafter called "CVT") used for a power transmitting apparatus of an automobile.
2. Description of the Prior Art
In the belt type CVT, a line pressure Pl is supplied to a hydraulic cylinder of an output side pulley.
The line pressure Pl should be controlled to a proper value of necessary minimum in which torque is transmitted by the belt without any troubles, and drive loss of an oil pump is to be avoided. FIG. 1 shows the relationship between a transmission-ratio .gamma. of a continuously variable transmission (.gamma.=Nin/Nout, provided Nin is the input side rotational speed and Nout is the output side rotational speed) and an ideal proper line pressure Plo, assuming an engine torque is T, as a parameter,provided T.sub.1 &gt;T.sub.2 with respect to the engine torque T. The more the engine torque T is increased and the transmission-ratio .gamma. is increased, the more the torque of the output side pulley is increased so that proper line pressure Plo is also increased.
In the prior line pressure controlling system producing such proper line pressure Plo, as shown in FIG. 2, a throttle pressure Pth as an increase function of the throttle position .theta. and a transmission-ratio pressure P.gamma. as a decrease function of the transmission-ratio .gamma. are produced as shown in FIG. 3 and used as pressures for controlling a line pressure producing valve. On a spool of the line pressure producing valve acts the following forces.
One directional force; EQU A1.multidot.Pl+(A2-A1).multidot.P.gamma. (1)
Other directional force; EQU A3.multidot.Pth+w (2)
provided A1, A2, A3 are cross-sectional areas of lands in the spool of the line pressure producing valve and w is a load on a spring. Thus, from the balance between both forces is established the following formulae. EQU A1.multidot.Pl+(A2-A1).multidot.P.gamma.=A3.multidot.Pth+w (3) EQU Pl=(A3/A1).multidot.Pth-[(A2-A1)/A1].multidot.P.gamma.+W/A1 (4)
While the proper line pressure Plo is defined by a curve as shown in FIG. 1, the line pressure Pl produced in the line pressure producing valve is a primary function of the throttle pressure Pth and the transmission-ratio pressure P.gamma. as shown in the formula (4) so that the line pressure Pl produced is difficult to coincide with the proper line pressure Plo.
When such produced line pressure Pl is applied to the actual CVT, the requirement of Pl&gt;Plo must be satisfied to ensure the torque transmission in CVT, so that when the produced line pressure Pl is adapted to a large engine torque T1 and a small engine torque T2 the produced line pressure Pl, i.e. the actual line pressure Pl becomes as shown in FIGS. 4 and 5. The slant lines show the difference between the actual line pressure Pl and the proper one Plo, and Pl is substantially increased, compared with Plo, when the engine torque T2 is small and the transmission-ratio .gamma. is large in FIG. 4 and when the engine torque T1 is large and the transmission-ratio .gamma. is small in FIG. 5.
That the actual line pressure Pl is substantially increased, compared with the proper line pressure Plo, not only degrades the durability of the belt due to the increase of tension of the belt, but also increases the drive loss of an oil pump to degrade the specific fuel consumption.